I added an electric wire to the bottom sections of my run. Although, my set up is not fort knox, I haven't had any issues yet.

Still new to poultry though. So, there is always that chance. A weasel could definitely get in, but not at night. My coops should be pretty fortified. I am using aviary netting over the run and I know if anything got up there, it would be heavy enough to bring the netting down. Hopefully, it never happens during the day.
But, with that said, we can only do so much. I read about those that have minimum security with never having a loss and then there are those at maximum security and can't figure out how a predator got in.

I think it is a gamble either way when raising poultry.