This app was mentioned in 1 comment, with an average of 14.00 upvotes
We have a DitecTV box exactly like that. You can actually control any TV as if you had a remote from anything connected to the wifi network. If the person memorized the key presses to get to the search function (trivial) they could do this from the street on their cell phone.
I'd check out the router logs to see if any suspicious devices have been connected. This is the most obvious solution to me.
Edit: this is the app I'm talking about https://play.google.com/store/apps/details?id=com.cognitial.directvremote
Edit 2: I'm an idiot, it's an ATT Uverse. Same thing, though. Here's the app: https://play.google.com/store/apps/details?id=com.att.android.uverse.easyremote