BRB p. 74, "Random movement":
Models with the USR "Random movement" move in the "Compulsory Movement" phase [sic]
BRB p. 24: "Compulsory Movement"
"The player has no control over their movement and thus they are considered compulsory." [sic]
Sorry for the [sic], only got the German
BRB at hand right now.
Units with Random Movement can garnison a building but have to leave it in the next turn because they are forced to move every turn.