How did Will Stutley get discovered in the Blue Boar in Chapter 4?

Will is discovered at the Blue Boar after a cut rubs against his leg and exposes his sword.

As Stutely sat thus, there came a great house cat and rubbed against his knee, raising his robe a palm's-breadth high. Stutely pushed his robe quickly down again, but the constable who commanded the Sheriffs men saw what had passed, and saw also fair Lincoln green beneath the friar's robe. He said nothing at the time, but communed within himself in this wise: "Yon is no friar of orders gray, and also, I wot, no honest yeoman goeth about in priest's garb, nor doth a thief go so for nought. Now I think in good sooth that is one of Robin Hood's own men."