Trapped chest? That is admittedly something I never considered, but it does stand to reason that it would apply a different routine since it is technically in battle. I will take a quick look and see if I can't find it.
...Here we go! Not quite what we were looking for, but I found the value that decides why it is whatever value is put into "Enemy Encounter" + C0.
This makes me wonder, is C0 the bit that decides that the byte will act as a Trapped Chest? If so that explains why they use a C0 here, if not and a whole byte is devoted to Trapped Chests, then this is a waste that we should keep it as 1C0+
$00/975E 69 C0 ADC #$C0 A:0015 X:0000 Y:0000 P:envMxdizc - Add C0 to Encounter Byte.
$00/9760 8D 00 18 STA $1800 [$00:1800] A:00D5 X:0000 Y:0000 P:eNvMxdizc - Store A in Encounter Byte.
$00/9763 A9 01 LDA #$01 A:00D5 X:0000 Y:0000 P:eNvMxdizc - Load 01 into second Encounter Byte (Underworld battles)
$00/9765 8D 01 18 STA $1801 [$00:1801] A:0001 X:0000 Y:0000 P:envMxdizc - Store A in Second Encounter Byte.
Now here's an odd part... the game checks what value is in 06C6 (It seems to be where Trapped Chest Items are stored at the start of battle) and then proceeds down an encounter list...
$00/8719 A5 C6 LDA $C6 [$00:06C6] A:002A X:0014 Y:0003 P:envMxdiZC - Load 06C6 (Location of Trapped Item) into A.
$00/871B D0 03 BNE $03 [$8720] A:005E X:0014 Y:0003 P:envMxdizC - Branch if not 00.
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$00/8720 20 39 87 JSR $8739 [$00:8739] A:005E X:0014 Y:0003 P:envMxdizC - Jump elsewhere.
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$00/8739 AE 00 18 LDX $1800 [$00:1800] A:005E X:0014 Y:0003 P:envMxdizC - Load Current Formation into A.
$00/873C E0 B7 01 CPX #$01B7 A:005E X:01D5 Y:0003 P:envMxdizC - Is it less than Golbez and Shadow Dragon?
$00/873F 90 0A BCC $0A [$874B] A:005E X:01D5 Y:0003 P:envMxdizC - If not, branch.
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$00/8741 E0 B9 01 CPX #$01B9 A:005E X:01D5 Y:0003 P:envMxdizC - Is it more than ?Goblin? Formation?
$00/8744 B0 05 BCS $05 [$874B] A:005E X:01D5 Y:0003 P:envMxdizC - If so, branch elsewhere.
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It should be noted that the formation between these two is the final Zeromus battle, but I can't reason what that deals with some trapped treasure items. It may fully be a "cancel item acquisition" though.
And there we go!
$01/D470 F0 1C BEQ $1C [$D48E] A:005E X:0000 Y:0000 P:envMxdIzc - If the Item in 1804 (Pending Item to give) is nothing than branch.
$01/D472 C9 54 CMP #$54 A:005E X:0000 Y:0000 P:envMxdIzc - Is it Arrows?
$01/D474 90 0E BCC $0E [$D484] A:005E X:0000 Y:0000 P:envMxdIzC - If less than Arrows, branch.
$01/D476 C9 60 CMP #$60 A:005E X:0000 Y:0000 P:envMxdIzC - Is it Shield or above?
$01/D478 B0 0A BCS $0A [$D484] A:005E X:0000 Y:0000 P:eNvMxdIzc - If greater than Shields, Branch.
$01/D47A 99 28 FF STA $FF28,y[$7E:FF28] A:005E X:0000 Y:0000 P:eNvMxdIzc - Store A in FF28 (Text to display)
$01/D47D A9 0A LDA #$0A A:005E X:0000 Y:0000 P:eNvMxdIzc - Load 10 into A.
$01/D47F 99 29 FF STA $FF29,y[$7E:FF29] A:000A X:0000 Y:0000 P:envMxdIzc - Store A in FF29 (Text Amount to display)
It's been a while but you should recognize this pattern, just another check for your Feature section, not to mention with this information and the older information of mine you could change how much is in a Bundle, if so desired.