I always cover my hands and make it as hard as possible to detect my PIN even when there's no one around for this reason. Skimmers are already small enough that

I always cover my hands and make it as hard as possible to detect my PIN even when there's no one around for this reason. Skimmers are already small enough that they can be fitted inside the machine, completely invisible from the outside. Cameras could be concealed in any number of places. The only rational thing to do is never trust that the machine you're putting your card into hasn't been tampered with.