Depending on how your terrestrial radio antenna is mounted, it may be possible to remove it and mount a small dual band antenna there and likely no one would notice. I tried that on my truck, but that area where the antenna mounts is on the plastic frame in front of the windshield, not down on the fender. There was no way to really get in there and swap the bases out.
I also removed my terrestrial radio antenna anyway (just unscrewed the element) as I have satellite radio and that was unnecessary. If you have that situation, there is always the option of drilling and installing a mount there on the fender.
Other than that, all your other (effective) antenna options are going to be somewhat less discreet. For an SUV, a jam mount or lip mount on the rear door/hatch would work, a 1/4 λ antenna is only going to be about 20" long for 2m. Even shorter for 70cm. Black mount and black antenna and it should blend in well.
A useful dual or tri-band antenna is going to be 30"+. At that point it's going to be hard to call it anything other than what it is.
Depending on who you are hiding this from, another option is mounting on the roof rack, if you have one. There are these motorized mounts designed for people that park in garages that lay the antenna down for parking. You could mount that and only raise the antenna when you are on the road and operating.
There are no good areas inside the vehicle, the body makes a moderately effective Faraday cage and the losses are brutal. I can't imagine anything hidden in a plastic bumper being very effective since VHF is pretty much line of sight and the mass of the vehicle is always going to block out 1/2 the sky.
A final option is a small mag mount or a window mount that you put up only when operating and take off and toss in the back when you aren't operating. Nothing permanent or too obvious. Again, this all depends on whom you are hiding from.
