[RPG] How could the enemies find the victims’ corpses left in a Magnificient Mansion

You find them as treasure!

Everyone likes gold and magical treasures, but a warlock or wizard prizes the treasure of knowledge above all. Ancient tomes of incantations, a scrap of lost research, a scroll encoding a single forgotten magical effect, a spell carved in ten-foot-high letters into solid rock in the fabled city of the Magus Confederation—these are treasure worth more than gold, because lost bits of knowledge cannot be bought at any price.

You can also get new-to-you spells from friendly spellcasters for barter or gold, or seize them from the bloodied books of defeated enemy spellcasters. These are equally treasure, though you might go about getting them differently than how you seek ancient spellcasting knowledge.

Short answer, they would be expelled too, unless you give the order to preserve it to one of the servant but the item will be lost forever.

Long answer:

Is the same mansion? No.

PHB 261:

You conjure an extradimensional dwelling in range that lasts for the duration.

As you can read it does not say you conjure a new or an existing mansion. Later on in the same description, it state that you can create but it does tell you anything about reusing the existing one. It also does not stop you to recreate an old one, but is not going to be the same one. Furthermore, at any point on the description says something about the servants remembering you, reinforcing the idea that is a totally new one.

You can create any floor plan you like

What happen to the item left?

Following the same logic, the spell description also state what the servant can do, under their limits. And since you can give things to humans, it is safe to assume that you can give to them items. You can order to keep the item, but as stated earlier the item will be lost.

Each servant can perform any task a normal human servant could perform, but they can’t attack or take any action that would directly harm another creature.

Now, what would happen to items that you leave in the mansion? The "mansion" does not hold ownership on any item that you have or leave behind. The same way you do not own any items in the mansion (you cannot take them out). So, unless you "give" to the mansion the ownership of an item, it would behave as if the Object is a Creature and drop it.

The servants can go anywhere in the mansion but can’t leave it. Furnishings and other objects created by this spell dissipate into smoke if removed from the mansion.

When the spell ends, any creatures inside the extradimensional space are expelled into the open spaces nearest to the entrance.

We can expand the ownership logic further. The nourishment of the food given by the spell does not disappear at the end of the spell or when you leave the mansion, but the furniture do. This mean that in order to take things out of the mansion they have to be given. This logic can be applied to your own stuffs. The "mansion" does not have any ownership of your stuff, hence, they are expelled from the mansion when the spell end (given that you did not give them to a servant).

It contains sufficient food to server a nine-course banquet for up to 100 people

Now, you might want to say: a creature is not an object. And it is true, but it is more of a mechanical way to delimit and categorize elements in the game. That would be a total debate but as far as your question goes is not that relevant. Furthermore, your DM is the one that has the last word, if you are the DM pick what would be more sensible for you.