depending on where it was placed, it could have been removed by the city as a sidewalk or mowing obstruction, or esp. if a local business or homeowner complained about it. If it was attached to a utility pole, then maybe the utility company. If it was a decent bike, maybe someone stole it. Is the one on W. 6th st past, couple blocks past lamar on north side still there? how about on s. congress or for the lady who was killed last week near 2222. There was supposedly oneat E.7th and chicon but it is gone if it ever was there.

I don't know who puts these up or even if there is a group doing it locally. it may just be friends of the killed rider. The ones put up by visual resistance in NYC are well maintained and include a sign explaining why they are there - visualresistance.org/wordpress/

something i might sugggest if you (or a friend) have time or interest in furthering this kind of advocacy, is to come up to the yellow bike project on e. 51st street THIS tuesday at 6.30 / 7pm, as there is a collective meeting. you could propose a ghost bike/sign making (and installing) project and see if they'll let you do it or help out.