Walking Crossbow

Infrared Vision. It is only hostile to creatures and objects it can see. The crossbow can only see creatures and objects that give off heat. It ceases being hostile whenever the creature or object has total cover.

Clumsy Design. The only action the crossbow can take is the attack action. Whenever the construct takes damage or makes an attack, roll a d4. on a 1, it falls on the ground prone. In order for the walking crossbow to get up from prone it must beat a DC 5 DEX save at the start of it's turn. On a success it can use half of its movement to stand up. Additionally, the walking crossbow automatically fails any check or save made to resist falling prone.
Lastly, whenever the crossbow is grappled or restrained in any way, the dm can decide whether or not the condition prevents the crossbow from making attacks.

Crossbow. Ranged Weapon Attack: +1 to hit, range 30 ft., 1 target. Hit: 3 (1d8 - 1)  piercing damage.

Alert. When not in combat, the walking crossbow is always holding its action to attack a creature or object it can see.