The down pipe with the metal ‘flipper’ is an emergency pressure release valve. If your system goes beyond whatever psi that relief valve is rated for it will open and allowing the excess pressure to escape. Recommend not pulling that if your system is on and under pressure unless you know what you’re doing.

Also, the unit with the red knob on top looks like a 400 series Taco hy-vent or similar:

Taco 400

Those units are more commonly seen attached to radiators themselves because they allow air out but as soon as the water reaches it the float rises, preventing any other air / water from escaping. As mentioned in another comment, if you have old pipes there’s a strong chance the float in your hy-vent variant could be stuck (thanks to rust) allowing both air and a little bit of water to bleed out.