Ideation: 'I was here'

'What value will transience add to the idea of "I was here"?'

An idea of when a person was there or a feeling of connection to said person

How to manifest it?

Type 1: Writing - Possibly on a wall and fading away. How could this be done electronically or non electronically? Water, air or a simulation of both on a screen or with a projector. 

The non electronic waterfall design has been modelled in google sketchup.

 Expanding on the model in the electronic version there could be no actual writing functionality , users would just touch the wall to leave there mark-there could be an element of anonymity versus ownership of the markings. There is also the question of how long the marks should be left there...

Where could something like this be placed? Hyde park for example...

Next: thinking about space: The 'thing' could be portable, semi-portable, fixed or environmental.

It doesn't have to be light, it could also be sound. Sound could mean that a sound gets quieter the longer it is there, or beeps which get further apart as time goes by. It could also be smell... The release of a smell which naturally gets weaker as time goes by especially in open spaces.