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Although all your lamps may be working, the bulb out sensor relay is a bit more complicated. It functions by comparing the current draw down the left and right side of each monitored circuit. If any circuit is unbalanced then the dash indicator lights, but that story has a few subtleties. One dead bulb on a monitored circuit will trip it -no problem. Two dead bulbs on the same ciruit (left and right both dead) and the indicator won't light -what are the odds? But it's also mildly sensitive and can misbehave even when all bulbs appear to be working. In that case it can be something a little obscure like a poor ground connection or a bit more subtle like mismatched bulbs (different brand or even different age). More often than not it would simply be a poor contact at one or more of the bulbs sockets, so that's the first thing to check. If everything else checks out it could be a failed bulb out relay. If a trailer wiring adapter has been added then that can also complicate the issue if it hasn't been done properly.
For U.S. cars of your year (w/o daytime running lights), the bulb out sensor only monitors a) the brake lights, b) the third (high) brake light, c) the rear tail (park) lights, d) the low beam headlights. For each circuit it's a simple left right current comparison, except the third brake light which is compared to an internal resistive current draw.
The first thing is to try to isolate the circuit causing the problem. If the indicator only lights when you touch the brakes then you know it's either a) or b) above. If you turn the park lights on and it lights then it's c) above. If you turn the headlamps on and it lights then it's d) above. Half the time, simply going around and giving a good thump to the associated lamp assembly will make the problem temporarily go away and tell your exactly which socket is acting up. If you can isolate it to one of these circuits then remove the related bulbs, clean the bulb contacts and the contacts in the bulb socket. If you see any signs of corrosion then do a thorough job and consider applying a little dielectric grease to the contact areas. Find the nearby ground connection and re-make it -usually just pulling the spade connector off the terminal and putting it back on or loosening and re-tightening a chassis screw will do the job. If you've isolated a circuit and problems persist then try swapping the bulbs left to right or buy new bulbs. If the problem is at the headlights then carefully check the connector to make sure the connector and contacts inside are not deformed -something that can easily happen if over-wattage high output bulbs are used.
If the problem is always there with all the bulbs off then it may well be the bulb out relay. It's in the relay tray behind the ash tray as noted in your owner's manual (available on-line at the Volvo USA website) -should be the round relay, leftmost in the back row. You may need to replace the relay. A used one would be the cheapest solution. These relays can often be resurrected by simply re-soldering the main contacts if you enjoy doing that sort of thing. If you want to confirm that it's a bad bulb out relay and don't have one to swap in for a test then more detailed fault tracing would be required.
Good luck and enjoy your first Volvo.
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Dave -940's, prev 740/240/140/120 Never overestimate what little I know.
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