I have a 2002 S60 that I bought used back in 2006. On the really hot days that we get here in Austin, I will occasionally come out to the car to find that the A/C fan is on. It normally happens when I've been driving it recently, like when I come out from buying grocieries. If I remember to turn off the fan BEFORE turning off the power, it won't come on. If I turn off the power first and then crank the fan knob to the left, it will still come on. If I turn off the power, crank the fan knob to the left, and then turn power back on briefly, the fan will power up and then back down, and it won't come on again. The fan turns off immediately when I turn the engine off, like it should, and some time between going inside to get groceries and coming back outside, it turns itself back on. This only happens over the summer, so it seems like this is intentional, but I can't find any documentation about it. Is this normal?
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1989 744 181k

perfectly normal, let it continue to turn on and don't try to out guess the electronics.

The ECC is programmed to dry off the evaporator core after the AC has been on. It does this by running the blower motor. If you don't let the evap dry, it will eventually corrode and will cost you a bundle to replace.

Check your cabin filter, it is supposed to be replaced annually.

Klaus
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Common sense trumps a PhD