Toddler has developed fears

It could be related to an experience that he went through somewhere, or witnessed happening. While growing up (even till now), I have a fear of not having the main door and gate locked - always checking several times whether it's been locked and being the one to close it has always been my most feared thing - because my grandma used to misplace our keys a lot, a couple of times it was near our unit or even from the door. That was how I developed the fear and while it doesn't happen so often now, sometimes I'd still get nightmares about people trying to break into our house! It could likely be something he came across in school, an advertisement somewhere, a children song with a video of mouses entering people's homes or noticing a mouse somewhere else - kids notice a lot more things than we do, including our surroundings and their parents' feelings! Having said that, I think it's something you have to address and help him get through the fear. If he is able to express his thoughts (since you mentioned that he thinks it's a mouse), you can carry him to the source of the strange noises and show it to him so that he's not as afraid after he knows what it is :) My son is afraid of sudden, loud noises that he cannot identify - but if I stay calm, keep him in my arms and within safe distance and explain it to him calmly then he'll understand it's not something to be afraid of :) While it wasn't triggered by anything, it's likely that it's just part of growing up where they start developing fears - and that's where we come in to help them understand things that aren't as scary as they think!