Football teams use lip readers? Do NFL teams fit espionage in their budget?

I once asked why NFL coaches always cover their mouths when calling plays and I was told so the other team doesn’t read his lips and know what play their calling.
When did this start and was there ever a case of someone getting caught reading lips? There would have to be a relatively high level of espionage going on here…knowing the coach called “Eagle 40” would be worthless unless you actually knew their playbook. However I feel like i’m reading way too much into this and it’s just a preventative measure.