This is, probably, the oldest and most persistent hydraulic myth in the world. A hydraulic cylinder is drifting - the piston seal must be leaking... I'm sure you've heard this one.
What makes things even worse is the fact that indeed, quite often, a leaking piston seal will cause a drift, but not for the reasons that a mechanic intuitively thought of when he decided to replace the seals (oil by-passing from blind end to the rod end), yet the fact remains - a piston seal is replaced and the cylinder stops drifting, and the myth is carried on until the next time the same mechanic spends six hours replacing an expensive OEM piston seal to find a cylinder drift just as before once it's re-installed.
I won't talk much about it - there's already more that enough information out there (here's a very nice video, for one), I just want to acknowledge the myth's existence and persistence.
When I am asked if a cylinder drift can be caused by a leaking piston seal - I say - Yes, but not always, and most likely not for the reasons that you're thinking!
A load holding/supporting setup is more than just a cylinder - it's a cylinder, that has an orientation in relation to the supported load, and the respective holding valve arrangement, therefore when you troubleshoot drifting load - you must consider the whole system (and not forget that when a rod goes in - oil must go out).