I had 4 Corydoras about a year ago - i found one dead a few months ago but still could only account for 2. i added 3 more about three months ago to make 5 but now i can only account for 3? i siphon the substrate once a week 50% and i provide caves for them to hide as they never seem to venture out other than feeding time. i never find any dead in the tank and i try as best i can to ensure they are not in the siphoned water - has anyone else had experiences like this. i can only assume that they died and just disintergrated in the tank. the only other fish i have are two dwarf gourami, 2 pakistani loaches, 4 cardinal tetra - 2 rummy nose (1 died and the other one is missing)
Heavily planted tank.

Ive had 1 go missing literally in the first night in the tank, dispite a complete tank strip down no trace of it was ever seen again...Wierd, it happens if its over a peroid of time like in your case then it very well could of been eaten/decomposed without you realise, or they could just be hiding

they could be dieing and getting eaten by the loaches.

or decomposing in your sand and you say heavy planted so it may jusy be hiding or dead in a hiding place