When you walk around in Tokyo you will find that many buildings have red-triangle markings on some of the windows / panels on the outside.

I noticed them as well but I could not think of an explanation. Digging for information brought up this:

Panels to fire access openings shall be indicated with either a red or orange triangle of equal sides (minimum 150mm on each side), which can be upright or inverted, on the external side of the wall and with the wordings “Firefighting Access – Do Not Obstruct” of at least 25mm height on the internal side.

Singapore Firefighting Guide 2018

The red triangles on the buildings/hotel windows in Japan are the rescue paths to be used in case of fire. All fire fighters know the meaning of this red triangle on the windows. Red in color makes it prominent, to be located easily by the fire fighters in case of a fire incident. During a fire incident, windows are generally broken to allow for smoke and other gases to come out of the building.

Triangles in Japan