They could be observing specific things about the planes sent to intercept them, such as their heat signatures and radar cross sections. But also, don't assume

They could be observing specific things about the planes sent to intercept them, such as their heat signatures and radar cross sections. But also, don't assume that the US was naive here. The USAF is pretty good at electronic warfare. It's very possible they were jamming the object's sensors in various ways.