How much you planted and WHEN did you plant ?
They deplete in time, after ~2-4 day nora plants die.

Also, if you harvest any plant below (1) cycles it will die.
For nora this means the incubator on that tile also dies and will not grow back (as it is not naturally grown).

It is important to note, that other players may also be finding your plants, and accidentally (or intentionally!) killing them.

Planting of high value plants, and sometimes low value plants should be done in a remote area so they are not discovered. Players other then yourself can and will harvest it, and if you make enemies they may even destroy them using brute force in order to deny you the materials.

Other then this plants can and will die under certain conditions, such as their life cycle which is 2-4 days I believe, or if harvested down to 0. They also can be destroyed by bullets that hit it even if it is just in the way of a target, so it is wise not to plant them near any NPCs as they may accidentally be destroyed.

as he is a PHM agent, i doubt he planted the stuff anywhere accessible by random other player to shoot/harvest it down.
As that, you should exclude that possibility.