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To the OP, in addition to what Art is providing, is your bulb out sensor a Kaehler or genuine Volvo? They are a different internal design. The cheaper Kaehler is all solid state. I’ve had similar goofy bulb out indicator problems with two brand new Kaehlers, one was an exchange after having problems with the first. When the second misbehaved similarly, I figured they might have been from a bad batch as others have not complained about them. My situation was slightly different in that the indicator would not pulse with the turn signals, but would occasionally illuminate for a short period. I could go a couple months without problems and there seemed no rhyme or reason for it to start again, sometimes touching the brake pedal, sometimes using a turn signal, sometimes just for the heck of it. I tried the usual swapping in new bulbs, cleaning contacts and grounds. Swapping between two cars and the problem moved, although one car was more sensitive. A Volvo one worked fine in either car. My only conclusion was that the two Kaehlers were too sensitive with the internal references that trigger detection of an unbalanced circuit being too close to the limits of normal lighting operation. Frustrated, I took the original failed Volvo one that couldn’t be fixed by re-soldering and turned it into a jumpered bypass sensor, permanently ending the problem.
Now having said all that, whenever turn signals create bizarre behaviour in time with the flasher or things change when touching the brake then it’s normally a bad or weak ground or electrical contact at one of the corner lamp assemblies (front or more often the back), but could also be in the bulb sensor. Typically what’s happening is the current passing through a lit bulb isn’t going straight to chassis ground, but is back feeding to ground through another route with a lower resistance, which is often through another bulb such as a brake light or park light, normally at the same assembly, but not always. If that other circuit is on the bulb sensor that can trigger a warning in time with the flash. If the bad ground or contact is simply weak and not broken then the problem may seem intermittent, often related to temperature and humidity. People who own or service trailers know these problems all too well.
If using the brake lights changes things for a while then what's likely happening is that the higher current running through the brake lights is making a short lasting change somewhere that improves conductivity, such as passing through a fractured filament and temporarily welding it, or is slightly warming and expanding contact metal somewhere making for a more solid contact, or is breaking through (micro-arcing) the corrosion at a bad contact or solder joint and temporarily improving conductivity.
It can also be a bad bulb. A dual filament bulb such as the turn signal may have a loose or broken filament that may be touching the other filament. Road vibrations can make this seem intermittent.
Because you have an issue involving turn signals, you can use that to your advantage. Swap left/right all brake and turn bulbs (front and rear). If the problem moves then that says a lot and you can try further isolation or simply proceed to my next suggestion. If the problem goes away then that says you have two or more bulbs that are not quite equal in the current they’re able to pass and when combined with other slight differences in wiring and components the left/right current draws are now more equal. Using different brands of bulbs left/right is often behind such issues.
Clean up all the contact points on all the bulbs possibly involved and their bulb holders. Make sure the bulb holder contacts are pressing solidly to both the bulb contacts and the light assembly. Remake all wiring connections at the lamp assemblies by pulling off connectors and reseating them firmly. Remake the chassis ground, loosen and re-tighten the screws. Use spray contact cleaner if you have it. Some, like me, like to use dielectric grease on all bulb socket contacts as a moisture and air barrier to prevent future oxidation, but others prefer to avoid it thinking that using a non-conductive lubricant compound somehow reduces the metal to metal contact, especially for some types of electronic connectors. Near as I can tell there is only anecdotal evidence to support that.
Somewhere in all the above likely lies your issue. If cleaning up the bulbs doesn’t help then focus your attention on the bulb sensor. I’ll periodically go through phases of such lighting issues and after a few attempts at trying to isolate it or if problems become variable then a total cleanup of the bulb lighting is my next step. Anything above and beyond fixing the problem at hand is considered preventative maintenance.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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