does any remember a similar thing in pokrmon red, blue and yellow? if u have surf already when u get to the s.s anne, you can surf behind it or sumthin to a little piece of land. and on it is a truck you cant open.

many rumors claim you can open it via a key u get somewhere from the mew/mewtwo scriptures in the secret lab (abandoned) next to blaine's gym(7th gym). i never found it though.

and of course inside is meant to be a mew at lv 100 and the hardest pokemon of all.

maybe there both just unfinished side quests. i woulnt be surprised if this trunk was also part of the unfinished way to resurrect Aeris.