搁 is the colloquial northern verb for 'to put / place'; standard equivalents are 放 or 放在. Erhua 哪儿 is Beijing-flavored. Southern speakers tend to say 哪里 instead.
grammar
Two readings in use: 搁哪儿 asking a placement question, and 搁哪儿了 asking where something already-placed ended up. The 了 signals completed action.
