What is making my tire squeak without riding it?

This can occur as the rubber compound deteriorates over time.

I personally had this problem this year when I used some MTB tyres that had been unused for 3 years. Fortunately for me i took it for a ride (being very cautious with grip levels) and when I was done, the squeaking was gone and grip was restored.

Brand new tyres come with a thin layer of mould-release compound on the outside of the rubber. This helps the tyre to come off the hot mould undamaged.

That means a brand new tyre has pretty bad wet-weather grip for the first 10 km of usage. Once its worn off, and the rubber has a bit of tooth to it, the slip is minimised. This is enhanced by dampness, so if your tyres are very new they can squeak on smooth surfaces.

Old tyres tend to oxidise and harden on the outside, along with brake pads. So its possible to have a fairly un-worn tyre that has a damaged thread and is breaking down. These can also squeak because of the small particles coming off the rubber - same reason a damp finger squeaks while washing glass.

The exactly same condition exists for new car tyres too.

UPSHOT Ride it for a bit, carefully, and reevaluate. If the tyres need replacing, then do so.