You may have seen a couple of odd names in the list of sensors for a bridge in Hue Log. Most likely they are of type CLIPGenericStatus and got names like the following:
These are not physical devices, like a motion sensor you put on the wall. They are virtual devices (i.e. they only exist in your Hue bridge) and are typically created by a Hue Labs features. You can tell by the model ID being HUELABSENUM or BEH_STATE.
This is a similar "leftover" as "Custom group for lights", and you may use the same method to clean-up as instructed on that page.