has the battery been recently changed? if it has and care was not present it is possible the abs fuse blew which is located on the abs module underneath the passenger front seat which needs to come out to gain access under the carpet.

if no electrical changes have occurred try this

the most likely problem causing the abs light to go is crud effecting the signal from one or both of the front wheel brake sensors and or the reluctor wheel attached to each hub. the abs system is pretty robust in a volvo. with a multimeter try this first.

Open a front sensor connector at or near the firewall (each side of the car has one). The resistance between the wires going to the sender should be .9-2.2 KOhms.

these sensors are held in place by a 5mm bolt BUT the sensor can be frozen into the hub because of accumulated carbonized brake dust and crud. if the sensor does not CHEERFULLY come out of its hole...STOP and go to plan B.

which is to.... regrettably..... remove the tire, remove the caliper and hang it out of the way WITHOUT disconnecting it.....remove the rotor exposing the reluctor wheel and sensor. CAREFULLY clean both. the wheel is fragile.... easily broken if you are careless or brutal. if one side is dirty likely the other is as well.

between a dirty reluctor wheel, dirty sensor and blown abs fuse imo this is 98% of the reasons why abs lights go on and stay on.