If you haven't seen it, it may not be a squirrel. It may be a large rat. You can get a large ground squirrel cage, also good for large rats, at your local hardware store. Just set it near the opening and bait it with a handful of peanuts in their shell.
Be patient, you will "miss" a few times most likely, but eventually they will trip the cage as they become more confident, even to the point of not having to bait the trap...if it is a squirrel.
Otherwise you should use a large sized sticky baited pad for a large rat, just outside the entrance. They will pass away before you realize they have been caught.
As was said, poison bait is your worst option as you can not control where they pass away.
I have caught some 35 to 40 large gray squirrels the last three years in my yard, including a large rat, in my single trap. I reocated the squirrels to a remote area rather than killing them. Squirrels are rodents, quite tough, and can be quite upset about being caught. They have huge claws and are very strong. Not pet-able or sweet as they appear to be when they accept food in the wild.
If you think you have a couple of gray squirrels near you, you actually have close to a dozen, but they all look very similar and are hard to differentiate.