The phrase "slow creep" means something that gradually progresses or advances over a long period of time, often in a sneaky or unnoticed way. Full definition
You notice the amounts on your credit bills slowing creeping up each month. (stockmonkeys.com)
Many times in training an alarm response isn't necessary, as training or life challenges encountered are slow creeping problems. (blog.trainheroic.com)
«They're doing the un-Target,» Antia says, citing the chain's slow creep into Canada. (canadianbusiness.com)