There is no actor in charge of surveilling the network. Every validator is. And every validator is incentivized to do so because you get a reward coming from the slash penalty. So everyone is checking on everyone else.
When a validator adds a block to the ledger it can include evidence of slashable offenses. Including evidence of a slashable offense gets rewarded so every validator is incentivized to include them.
There is no separate notion of a slasher, everyone gets to provide slashing evidence when they build a block. I don't understand why you build such a weak straw-man and the push it forward as if it was the truth without even checking how the system operates.
