My 2001 S-60's battery has drained 5 different times when my parking lot attendant leaves the key in the ignition (with everything turned off). Has anyone else had this experience? My mechanic says this cannot possibly happen, and the dealer says there is no electrical problem.

With the S60, S80, and new S70, leaving the key in the ignition is leaving the lights on. Inserting the key turns on a radio transmitter that interrogates the transponder embedded in the key. It's all part of the anti-theft system.
It is important, when you use Valet parking, to inform the attendant. Most of the Valets at hotels already know about new Volvos, and will remove the keys, but restaurants are another story. You will end up with a dead battery every time.

Dealers should know about this; Volvo issued internal bulletins beginning with the '99 S80. You can find them listed on the VCNA web site.

You're being hosed as I've seen problems with electrical drains like yours. Something is causing an excessive current draw and the ONLY way it can be tested is by using a VOM and checking the exact current draw with known specs. Take it back to your dealer and insist that they do that as it's possible and does happen with some new Volvos, rare but happens.

I admit that I failed to note the part about the "key being left in the ignition" and as noted above, yes.....DON'T do that! That in itself WILL cause an accesive current draw on many newer cars and IS causing the problem. Simply tell the guy doing that to remove the key from your ignition and you should be fine.