It's time to think at introducing a global game clock to drive animations.

Blood spread on floor is a good example to explain its interest:

A blood print vanishes after 'some' seconds. Currently a counter of seconds is
set, decremented at each frame, until it comes to 0.

To avoid to have to scan all 'animated' objects, their 'counters' are changed
only when those objects are rendered, that is when they are visible.

For blood spread on a level, it means that if the player leaves that level and
come back several minutes later, all the blood prints will start to vanish only
at that time.

So, instead of decrementing a counter, we rather need to compare its value with
a current global clock.

The value of that global clock has to be saved in the savegame.

We have game time on Michelangelo dialog, 
don't we?
So we add the game time at insertion and 
check if its already time to delete 
compared to game time.

If I understood correctly.

Game clock is not really the 'game time'.
The 'game clock' can be stopped for instance when running the takeover
mini-game, while the 'game time' must continue to run.

My post is a self-reminder to something we should work on in the near future,
rather than a description of what to do.

Sounds like a good idea to me.