Good catch. It goes to the 5th floor of the department store. That map might have been changed during development.
CeladonCityObject: ; 0x18022 (size=189)
db $f ; border tile
db $d ; warps
db $d, $8, $0, CELADON_MART_1
db $d, $a, $2, CELADON_MART_1
db $9, $18, $0, CELADON_MANSION_1
db $3, $18, $2, CELADON_MANSION_1
db $3, $19, $2, CELADON_MANSION_1
db $9, $29, $0, CELADON_POKECENTER
db $1b, $c, $0, CELADON_GYM
db $13, $1c, $0, GAME_CORNER
db $13, $27, $0, CELADON_MART_5 ; ←
db $13, $21, $0, CELADONPRIZE_ROOM
db $1b, $1f, $0, CELADON_DINER
db $1b, $23, $0, CELADON_HOUSE
db $1b, $2b, $0, CELADON_HOTEL
On a related note, Celadon Mart 5F is located away from the other floors in the game's internal ordering. More evidence that this map was changed during development, and only added to the department store at the last minute.
...
HALL_OF_FAME EQU $76
UNDERGROUND_PATH_NS EQU $77
CHAMPIONS_ROOM EQU $78
UNDERGROUND_PATH_WE EQU $79
CELADON_MART_1 EQU $7A ; ←
CELADON_MART_2 EQU $7B ; ←
CELADON_MART_3 EQU $7C ; ←
CELADON_MART_4 EQU $7D ; ←
CELADON_MART_ROOF EQU $7E ; ←
CELADON_MART_ELEVATOR EQU $7F ; ←
CELADON_MANSION_1 EQU $80
CELADON_MANSION_2 EQU $81
CELADON_MANSION_3 EQU $82
CELADON_MANSION_4 EQU $83
CELADON_MANSION_5 EQU $84
CELADON_POKECENTER EQU $85
CELADON_GYM EQU $86
GAME_CORNER EQU $87
CELADON_MART_5 EQU $88 ; ←
CELADONPRIZE_ROOM EQU $89
CELADON_DINER EQU $8A
...