I think they are enchantment bonuses but I do agree they both don't stack.
I agree with the Hat of Anonymity, Camouflage Paint (hopefully your being paid for your assassinations and this is a cheap +5 bonus on each kill).
QUOTE=The Gilded Duke;7574132]Stacked Psionitrix of Chameleon (Complete Psionics) can get a bit expensive, but gives you a +10 enhancement bonus to hide. Continuous Collar of Umbral Metamorphosis (Tome of Magic) gives you hide in plain sight and +8 racial bonus to hide. Modify one of the Crystal Masks (Expanded Psionics Handbook, SRD) to get a +10 competence bonus to hide. Get a MW tool of hide (DMG) to get an always on +2 circumstance bonus to hide. From those right there you get +30 to hide.[/QUOTE]
I dont think the DM will allow a MW item for a +2 to hide because I can't think of anything other than a gillie suit idea that would allow that bonus.
The bonus is either +6 or +8 because I am running a character with that template unfortunately its a +1 LA due to it being a PC and not an NPC or random monster. So that may rule out this idea but if you can do it and pay off the LA with the rules from U.A.