It’s not like you are going to find one lying on the ground somewhere. A stellar-to-planetary mass black hole that found its way into our solar system would disturb the orbits of the planets, asteroids and comets. It would be potentially cataclysmic if it got near the inner solar system. A brush with the Opik-Oort cloud or the Kuiper belt alone could set off a storm of comet bombardment (see Nemesis hypothesis)
A sub-planetary mass black hole isn’t “black” due to Hawking radiation. The lighter they get, the smaller the event horizon and less gravitational influence around it, but the greater the emitted power of the Hawking radiation. A truly small black hole weighing as much as a mountain would appear as a very painfully bright spark as its event horizon is at a tremendous effective temperature. Such an object is far too dense to be supported on the surface of the Earth by normal matter. It would drop right through and oscillate around the center of the earth.
A black hole lighter than a mountain is a very dangerous thing. As it looses mass through emitting Hawking radiation, it gets hotter and thus it looses mass ever faster. It is believed such an object will explode and disappear as it reaches zero mass.