One of my P38 headlights has a little water sitting in it after rain - passenger side - haven't tried to get in it but is this a replacement job - reflector is showing signs of corrosion !

The seal around the glass i'd have my money on!!

Or water has comehow come through the boot on the back where the bulb comes through..

You could try separating the glass and resealing it?

You can repaint the reflector with chrome effect paint, atleast thats what i did with my Dads old V70 and it worked..

drill a hole for drainage? (at the bottom..)

Sorry to turn up like a bad penny but I was told by my headlights' refurbisher that the Valeo lamps for the P38 are not of easy disassembly with the chances of breaking the glass very high, because of the way the sealant is "embedded" in the housing, he actually refused to do it.
He rarely turns down a challenge, so I do not doubt he knows what he is talking about.
If you do manage though, I'll keep in mind your success to have mine refurb.

In the meantime I'd stick with the hole idea ....