Maxperson
Morkus from Orkus
This looks more like an investigation check than a perception one. As you said "I wouldn't make that a part of stealth, though if they make an appropriate DC perception check, I'd point out the discoloration and let the PCs investigate from there if they wish." All i'm saying is may be you wouldn't, but it could if a DM want, use Stealth rules just as they are.
It's not investigation. Noticing an oddity is pure perception. What happens afterwards to figure out the oddity would be investigation, though I seriously doubt a roll would be required to figure out that an invisible golem is in the corner. Just about anything they do will reveal that a large, invisible object is there.