If they're scanning plates, they might still notice that you've been there more than the maximum time. For example, you feed the meter for 30 minutes at 1pm. They scan your plate at 1:05pm. You re-fill via app at 1:30pm. They scan your plate at 1:40pm and note that you were already there at 1:05pm and therefore have been parking for 35 minutes, above the 30 minute maximum.

If they were really smart, the apps could literally tell them you were there over time. They scan your plate at 1:05pm, the app notes that your plate paid for additional parking at 1:30pm, the app lets parking enforcement know that they can go and ticket your car in 5 minutes.