There's something in there!

We have a little old shed down the drive, and the dogs have decided that there is something in there for them to chase.
The netting over the old doorway, is what I put up when they weren't very old, as they liked getting in there to find things. But as there was stuff in there that we didn't want them getting into (fertilizer etc) I put netting over all  the doors and old windows frames, as the glass had all gone from the windows.
This has kept them out all this time, but I think there could be a possum - or rats - that have taken up residence.