My toilet will fill the tank for a few seconds a few times a day. This occurs with out a flush.
What could cause this ?

A leaking flush valve (flapper).

A leaky flapper. Sometimes you can take a piece of sandpaper and slightly go around porcelain hole a couple of times or just clean off bottom of flapper itself. Try this before buying new flapper. Or chain could be too tight, holding flapper up.

Like Hkstroud says your problem is either the flapper valve or the flush valve seat where the flapper valve sets, inspect both for distortion, clean the flush valve seat with 00 steel wool check if it is fixed, if not replace the flapper valve if that does not help replace the flush valve it is cheap and easy to replace, good luck.

John

Like Hkstroud says your problem is either the flapper valve or the flush valve seat where the flapper valve sets, inspect the flapper valve and the flush valve seat for any distortion, clean the flush valve seat with 00 steel wool if that would not help replace the flapper valve if problem still not fixed replace the flushvalve it is cheap and easy to replace, good luck.

John