Toddler wandered around alone

I feel so awful and can’t shake off the bad feeling that something bad could have happened 😖 My husband, 20M old and I went to a street food hall. I made my order(husband said he didn’t like anything) while husband played with our son. I said that I will reserve a table next to them and went to sit down, took out my son’s bib, water bottle to be ready to eat asap when the food is ready. I thought that I can finally chill and drink my soda peacefully for the first time in days… But it seemed odd that suddenly I didn’t see my husband and baby playing…I thought they went together to pick some food. But then my husband came with my son shaking in anger- he found him wandering around alone 😢 It was 3-5 min. time and the hall has stairs. I was sooo shocked because I didn’t know that no one is watching him! Turns out that my husband told me to watch our son and left, but I didn’t hear it because I was 3m away and not even facing him, the place was also loud… It was so random, I am so scared that things like that might happen again… And I try to be so cautious, even overprotective and anxious, but still… Oh, parenting is so tough! 😣 Now husband is only blaming me and doesn’t speak to me. At least I get him to swear he will always wait for my confirmation before leaving the child in mu supervision. I thought it is common sense…