Quite possibly just displacing a small amount of fluid as it expands as you have overfilled the system.

If you filled it right up to the cap, it was technically over filled, and will displace a few ml when the fluid expands (remember the fluid will expand with any het, not just due to overheating). Assuming no major issues at play the vacuum will return the majority of the fluid back to the machine when it cools down.

You are likely observing something that has always been happening.

For peace of mind, give everything the once over to make sure it is not getting into the cylinder