@HaoTran92 Yes, it could mean that you're about to go out. But I honestly don't think that is used often. It's more common to say, "I'm about to leave" or "I'm on my way." The more common context to use "almost out" would be for running out of an item. Ex: "I'm almost out of time."
almost out means like you are running out of something. Example: “I’m almost out of cookies!” You could say “I’m almost out of the store” but it’s better to say “I’m leaving the store now” I hope my answer helped!