I've started noticing this as well. Except mine does it just with me hitting the power button to get it to unlock. What ROMs have you tried maybe its one of them?
I'm using EclipseTW. Thought about getting a replacement because of it but don't want to risk getting an ME7 phone.
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Phone: Samsung Galaxy S4 - Rooted, TWRP, Hyperdrive ROM
Tablet: Nexus 7 8gb - Unlocked, Rooted, Stock 4.3; Nexus 7 2013 16gb - Rooted, Unlocked, Waiting for Official 4.4
Past Phones: Samsung Galaxy Nexus - Unlocked, Rooted, ParanoidAndroid3; Motorola Droid Bionic - Rooted, Stock 6.7.232; LG Revolution - Rooted, Stock