If you stumble upon a situation where your ex happened to kiss you, it can mean several things. If it’s merely a friendly peck on the cheek, it just means they still have you in their good books, and they still care about you but don’t necessarily want to get back with you.
On the other hand, if your ex goes for the lips, that’s a very open invitation to get back to the relationship and try to work things out this time. All of this is answered under the pretense that you’re currently single and not seeing someone else.