Such a facility would definitely need regular maintenance.

And for a truly intelligent prisoner the weakest spot is not prison walls or the door, it's the keeper with the keys in his pocket.

The range is only a few meters. The bigger problem is people bringing compromised devices within range. Limestone won’t stop that.

When I was working with SQuIDs, I couldn't even have keys in my pocket or wear a belt buckle. That was low-key compared with the bio clean room whose denizens showered at entry, but devices are a simpler matter of protocol.