The serial number can be located in several different places depending on the product model.
 

Handheld Cameras

Cx-Series:  Located under the flap on the side, near the USB port.
Ex-Series:  Located under the battery on the bottom of the camera.
Exx-Series:  Located inside the battery compartment under the battery.
Txxx/Bxxx-Series:  Located on the inside of the door that covers the ports and SD card.
T6xx-Series:  Located inside the battery compartment under the battery.
GF-Series:  Located under the battery (the battery has to be removed).
i3/i5/i7:  Located inside the battery compartment under the battery.
ixx/bxx-Series, InfraCAM, BCAM:  Located on the left side of camera under the rubber cover.
B/P/SC600-Series:  Located under the battery (the battery has to be removed).
PC software:  Located below the barcode on the software packaging (SW S/N).

Automation/Science Cameras

A400/A500/A700:  Located on the bottom of the camera.
A40/A50/A70:  Located on the bottom of the camera.
AX8:  Located on the side of the camera near the bar code.
A35/A65:  Located on the bottom of the camera near the bar code.

Firefighting Cameras

Kx/Kxx-Series:  Located near the USB connector, under the rubber flap near the front of the camera.

Automation and Science cameras

AX8: Bottom of the camera, near the bar code

Ax5: Bottom of the camera, near the bar code

A3xx: Bottom of the camera, near the bar code

A6xx: Bottom of the camera, near the bar code

A6xxx: Bottom of the camera

A8xxx: Bottom of the camera

Xxxxx: Bottom of the camera

Handheld Thermography cameras

FLIR One: Next to the phone/tablet connector

C-series (pre-2020): Under the USB port cap

C-series (2020): Bottom edge, right side when facing to the camera screen, next to the tripod hole

Ex-series: Under the battery on the bottom of the camera

Exx-series (pre-2017, Ex0): Inside the battery compartment under the battery

Exx-series (2017 and 2020-series, Ex5 or Ex6): Under the battery on the bottom of the camera

Txxx/Bxxx-series: On the inside of the door that covers the ports and SD card

T5xx-series: Under the battery on the bottom of the camera

T6xx-series: Inside the battery compartment under the battery

T8xx-series: Under the battery on the bottom of the camera

T10xx-series: Inside the battery compartment under the battery

i3/i5/i7:  Inside the battery compartment under the battery

ixx/bxx-Series, InfraCAM, BCAM:   On the left side of the camera, under the rubber cover

B/P/SC600-series: Under the battery (the battery has to be removed)

Test & Measurement equipment

TG1xx: On the left side, under the FLIR Logo

TG130: Inside the battery compartment under the battery

TG2xx: Next to the USB-C port, in the top of the unit

MR1xx: On the side of the device

TG5x: On the head of the unit

MRxx: On the side or back

Firefighting cameras

Kx: Near the USB connector, under the rubber flap near the front of the camera.

Kxx_ Near the USB connector, under the rubber flap near the front of the camera.

Optical Gas Imaging cameras

GF-Series: Under the battery (the battery has to be removed)

G-Series : Under the battery (the battery has to be removed)