Does my child notice when I step away to calm down?

Yes, and rarely as absence alone. What registers most is the state you're in when you come back.

A parent who steps out at a rolling boil and steps back in able to talk has demonstrated the entire skill without narrating a syllable of it. Of the three channels Amanda Sheffield Morris and colleagues identified in their 2007 review, the one that carries most of this is plain observation, which runs whether or not you intend it to.

So the break isn't a gap in the parenting. It's one of the few moments where your child sees regulation performed instead of described.

Leaving and returning are one move, and it's separating them that makes the first half look like abandonment. A young child does need the second half to arrive promptly and predictably. I'm going to the kitchen for two minutes, then I'm coming back costs you nothing and removes the ambiguity completely.

What makes an exit land badly isn't that it happened. It's an exit with no stated end, or one delivered as a punishment, the door closing on a child who's just been told they caused it. That isn't regulation, it's a sanction, and a child sorts the two immediately. The adult-partner version of this line is worked out in Take a break without stonewalling; with a child, the stated return time does more work and the explanation does less.

Something ordinary, ideally. The return doesn't need a speech. Walking back in and being normal is most of the content, and a child reads it immediately.

If there's something to say about what happened before you left, that's a separate conversation with its own timing: How to tell your child they hurt you. It doesn't have to be the first thing out of your mouth, and it doesn't have to be today.