  We present multiwavelength (optical/near infrared/millimetre) observations of
a short duration gamma-ray burst detected by Swift (GRB 050509b) collected
between 0 seconds and ~18.8 days after the event. No optical, near infrared or
millimetre emission has been detected in spite of the well localised X-ray
afterglow, confirming the elusiveness of the short duration events. We also
discuss the possibility of the burst being located in a cluster of galaxies at
z= 0.225 or beyond. In the former case, the spectral energy distribution of the
neighbouring, potential host galaxy, favours a system harbouring an evolved
dominant stellar population (age ~360 Myr), unlike most long duration GRB host
galaxies observed so far, i.e. thus giving support to a compact binary merger
origin. Any underlying supernova that could be associated with this particular
event should have been at least 3 magnitudes fainter than the type Ib/c SN
1998bw and 2.3 magnitudes fainter than a typical type Ia SN.
