I say, gently remove the vertical bricks on the side of the wall and see what is behind them? It appears to me, they may have been removed before and then put back in? They seem sloppily installed, or re-installed and the rest of the wall seems firm and intact? The metal detector that was used, what kind was it? Ground penetrating radar, deep search detector, or just a regular everyday metal detector? There could be metal supports in the walls, possibly at the bottom and top and possibly at each corner? If so, they would definitely indicate that something metal was behind the wall to a regular and possibly deep search detector, depending on the kind used and indicate treasure, where there is none, just metal supports?
Where the wall turns and has a column and a curb? Someone who said an underground sewage system, could be right, but don't believe it is a septic tank? Could possibly be a cistern though? And then, we could all be wrong and it is a hidden treasure, waiting to be discovered? Removing those vertical bricks might reveal, what, if anything, other than more mud, is behind that wall? Interesting what you said about the stones, the heart shaped ones, being an indicator of Japanese treasure? I would sure go to the library and look that one up! And while you are at it, look up the Philippine Treasure Recovery Laws and find out if you, in the future decide to give up and turn it over to the Government for recovery and treasure is recovered, how much, if any of it, you would be entitled to, as the discoverer of the site, where the Treasure is recovered? Those 5-6 vertical bricks, sure do intrigue me though! Afraid I would have to have a look there first thing? Anyway, good luck friend! Hope you find what you are looking for!