It could be a abs pick up ring!
The abs pick up ring just looks like a cog. The abs sensor works like a magnet and it detects the gaps in the abs ring when the wheel is turing, like the gaps on a cog in between the teeth.
If the abs cannot detect this it triggers the abs, this can be cause by a corroded abs pick up ring or a cracked one!
The fault can be intermittent and won't put your abs light on!
A simple visual check of them will tell you if they are ok or not!