Could U-boats report that escorts were disappearing?

Could a U-boat count enough escorts to notice that convoys were becoming more lightly protected?

Yes. Contact and post-patrol reports provided useful estimates even though each boat's visual range was limited.

A submerged boat could not see an entire convoy system at once, but its crew could still observe and estimate the escorts around a contact. U-boats sent extensive contact reports while operating. Commanders who returned also produced detailed accounts of what they had encountered. Those reports incidentally helped Britain locate transmitting submarines, but they also gave Dönitz the raw information needed to notice thinning escort numbers. The intelligence failure therefore occurred at headquarters as much as at sea.