I think you might have confused the calibration set up in the first step of the directions. This must be done each time the unit is powered on, placed on a different wall, or whenever it is providing ambiguous indications. For you convenience, I have pasted the operation manual below. You should download and review the entire manual - free of charge here
1) Hold the Stud Sensor 50 flat against the surface making firm contact.
2) Press in and hold the activation button. The green
and red LED's will light. When the red LED goes off,
the unit is calibrated
. Keep holding the activation button
during all of the following procedures.
Note: While calibrating, the Stud Sensor 50 must not
be placed directly over a stud, dense material such as
metal, or over a wet or newly painted area, or it will not
properly calibrate. If the Red LED stays lit, move to
a different location and try again.
2) Slide the Stud Sensor 50 slowly across the surface
in a straight line. As it detects a stud edge, the red LED
will go on.
Use the handy pencil notch located at the top of the
unit to mark the stud edge.
3) Repeat the above steps from the other side of
the stud. Coming from the opposite direction, mark
the other edge of the stud. The midpoint of the two
marks indicates the stud center.
Cautions on Operating
The proximity of electrical wiring or pipes to the surface
of the wall may cause the Stud Sensor 50 to detect
them as studs. You should always use caution when
nailing, cutting or drilling in walls, ceilings and floors
that may contain these items. Always remember that
studs or joists are normally spaced 16 inches or 24
inches apart and are 1-1/2 inches in width. To avoid
surprises be aware that anything closer together or of a different width may not be a stud.
I hope this helps & good luck!