'The only exception to this are mutants, which are STM and must be roleplayed unless agreed upon otherwise, though they can be STK'ed if they are in a Major Faction Base, or easily visible from it.'
Taken for 7.0b in the Stalker RP Rules and Regulations, which is stickied in the main SRP thread. Bloodsuckers within a few feet of a base are up for STK, along with those inside the base unless you have authorization to do so from Thy or a Server Supervisor.
With that out of the way, you're both in the wrong. Slendermang should have stayed in character and not use the physdesc to see your position, and you shouldn't have been that close to the base. In the future stay away from major faction bases, and those dealing with mutants need to learn to take the fact they're cloaked as they are INVISABLE, even if you can see their OOC physdesc floating above their heads.
Edit: Appearantly it wasn't Slender. Whatever, what I've said still has a place here.