library(odbc)
library(DBI)

# dbListTables in a function: gives a warning message

dbListTablesTest <- function(dsn, userName, password){

  con <- dbConnect(
    odbc::odbc(),
    dsn      = dsn,
    UID      = userName,
    PWD      = password,
    Port     = 1433,
    encoding = "latin1"
  )

  availableTables <- dbListTables(con)
}

availableTables <-
  dbListTablesTest(
    dsn = "myDsn"
    ,userName = myLogin
    ,password = myPassword
  )

# dbListTables not within a function works fine (no warnings)

con2 <- dbConnect(
  odbc::odbc(),
  dsn      = "myDsn",
  UID      = myLogin,
  PWD      = myPassword,
  Port     = 1433,
  encoding = "latin1"
)

availableTables <- dbListTables(con2)

warning messages from top-level task callback '1'
Warning message:
Could not notify connection observer. trying to get slot "info" from an object of a basic class ("character") with no slots

R version 3.4.2 (2017-09-28)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1

Matrix products: default

locale:
[1] LC_COLLATE=Dutch_Belgium.1252  LC_CTYPE=Dutch_Belgium.1252    LC_MONETARY=Dutch_Belgium.1252 LC_NUMERIC=C                   LC_TIME=Dutch_Belgium.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  tools     methods   base

other attached packages:
[1] DBI_0.7    odbc_1.1.3

loaded via a namespace (and not attached):
[1] bit_1.1-12     compiler_3.4.2 hms_0.3        tibble_1.3.4   Rcpp_0.12.13   bit64_0.9-7    blob_1.1.0     rlang_0.1.2