” but I keep them inside those protective covers”
Made from tin foil, right?
Mylar is my guess. Maybe aluminum.
I have seen way too many movies where tech savvy folks clone phones via proximity or steal credit card info with some sort of slide-thru reader. I have watched my country become a national security state, with ubiquitous cameras, drones and, yes, RFID readers. I think that if there is sufficient cause for paranoia, it is no longer paranoia. Note that these protective sleeves are available on the open market,implying a relatively large perceived need.
YMMV. Since I have very little understanding of actual technological capabilities, I opt for caution.