Question: what does it mean when solve returns just the variable itself when solving equation for the variable?

When doing solve(res,x); maple returns back x

Does this means the equation is true for any x?

Also, doing PDEtools:-Solve(res,x) returns nothing.

I was trying to check if my solution is valid for an ode, and I obtained res above is what odetest() gives back, I wanted to find for what x could res be zero.

Here is the code

restart;
ode:=diff(y(x),x)=2*sqrt(1+y(x))*cos(x);
my_sol:=y(x)=sin(x)^2+2*sin(x);
res:=odetest(my_sol,ode);
res:=simplify(res)=0;
solve(res,x);

And now 

PDEtools:-Solve(res,x)

gives nothing.

So I am a little confused what is going on. does solve result mean the equation is true for any x (I think the solution I have is correct, so may be what this means,  but wanted to make sure, as I do not think I've seen this before)  And if so, why then PDEtools:-Solve did not return the same result.

Maple 2020.2, windows 10, Physics 897