Once you have filled the Weapons crate with the 5 Necronium 2h greataxes, as requested by the Suspicious note, you will have left it on the tree stump behind the Gem shop in Falador.
A cutscene will play where you are knocked out by a monkey. Searching the crate, you will find this note.
Below is the text you can read from within the message.
Thank you for helping further our cause. Your efforts have been noted.
P.S. Sorry about knocking you out - no hard feelings?
After completion of the associated quest, if you have unlocked the entire Hub track, this item can be retrieved from the Quest Item Storage chest at the Quest Point Caravan if lost or destroyed.
There is no need to keep this item after the miniquest has been completed. As such, it can be destroyed or kept as you desire.